Arboretum heralds release of new USDA Plant Hardiness Zone Map

January 26, 2012

This week, the US Department of Agriculture unveiled its new Plant Hardiness Zone Map (PHZM), a development that has been long anticipated by gardeners and researchers. Like its earlier incarnations, the new PHZM provides guidelines to predict a region’s average annual minimum temperature, a vital statistic in determining whether or not a plant may survive the winter in a particular area. Last updated in 1990, the map now features a number of significant updates. For one, it has gained interactivity through a Geographic Information System (GIS) that enables users to zoom in at regional and state levels; it also has a tool to identify a zone by zip code. The quantity and quality of the data represent another marked improvement— the model utilizes 30 years (1976–2005) of data and a wider geographic sampling of weather station data. Lastly, some highly sophisticated algorithms facilitated the analysis, interpreting local weather station data as well as such geographic characteristics as elevation, proximity to bodies of water, and terrain.

Arnold Arboretum Curator of Living Collections Michael Dosmann served as a technical reviewer for the new PHZM, and applauds the finished product. “The new map is fantastic,” he remarks. “I think it’s a tool that all of us in the horticultural and botanical communities will find practical and trustworthy.” While a broader array of weather stations contributed data this time around, it is the longer time span of data collection that impresses him most. “To me, 13 years [the period of data collection used for the 1990 version] is not long enough to confidently predict just how cold it might get in your area. When forecasting whether or not a plant will survive, it pays to be somewhat conservative.” Dosmann explained that the boundaries drawn by the PHZM are not based on winter’s average temperature, nor its duration, but on the extreme cold temperature event for each winter. “People often forget this part of the equation. If on average your thermometer dips to between 0°F and -10°F every year, you are still in Zone 6 despite how the rest of the winter’s temperatures shake out.”

In the new PHZM, many zones have shifted from their previous designation, typically about a half-zone warmer. Does this provide additional evidence of climate change? “While we see upward movement in some zones, a few areas actually dipped into a colder zone,” advises Dosmann. “Zone creep, in either direction, can be attributed to a variety of things. Since a broad dataset was used to draw the map, scientists have more variables to consider in determining how these changes came about.” Additionally, in urban and suburban regions, the cities themselves may be greatly influencing temperature. “We know urbanization creates heat islands, making them significantly warmer than their rural surroundings. So we shouldn’t be surprised when the average annual minimum temperature in and around cities changes over time.”

Looking at the interplay between plant hardiness and geography is nothing new to the Arboretum. In 1927, Arboretum taxonomist Alfred Rehder first published a map of climatic zones across the US. As horticulturists began to cultivate newly discovered plants across the country, reports of their survival were used to assign hardiness ranges and ratings to these species. Although the Arboretum continued to update and publish its map until 1971, it was the USDA’s PHZM (first issued in 1960) that became the industry standard. Inspired by the Arboretum’s system, the USDA map factored a broader array of data as well as more sophisticated analytical tools.

Recently, the Arboretum has focused more technology on recording its local weather events, deploying an array of small weather stations across the landscape in 2008 and establishing a new permanent weather station a few months ago. Collected data allow Arboretum staff to better document conditions in the landscape, and to even identify microclimates, or small fluctuations in climate, due to terrain, aspect, or proximity to buildings. While overall the Arboretum landscape is nestled in Zone 6b (-5° to 0°F), there are a number of microclimates that offer slightly warmer lows. This gives curatorial and horticultural staff areas to cultivate plant species that are more tender and would typically perform best in Zone 7.